Ah, well some Procs occur on a tick basis. I believe VM is one of those. There are several "ticks" that occur over a time period and each "tick" has a chance. For example: Sub system attacks with a beam has 4 beam hits and each has a sub system attack chance. Therefore 4 ticks. BFI I believe has a similar "tick".

Interestingly enough, 4 ticks on a doff with 20% nominal proc rate will give a effective proc rate of 59% for said doff. If i had to guess though, I would say maybe it rolls once for each eligible subsystem, which would be 3 "ticks" for the doff. This would give an effective proc rate of 48.8%. Assuming any individual doff could only proc once for simplicity, this would give a 86.58% change of at least one proc occurring, with the following breakdown: